1. Field of the Invention
This invention relates to superconductor integrated circuits, and in particular provides an etch stop for the etching of molybdenum resistors in superconductor integrated circuits.
2. Description of Related Art
Superconductor integrated circuits using Josephson tunnel junction devices have been described in a number of U.S. Pat. Nos. (e.g. 4,430,662 issued February 7, 1984 and 4,554,567 issued Nov. 19, 1985, both to Jillie and Smith and both describing integrated circuitry and methods of manufacture, Josephson junction device configurations are also shown in U.S. Pat. Nos. 4,423,430 issued Dec. 27, 1983 to Hasuo et al. and 4,421,785 issued Dec. 20, 1983 to Kroger). The usage of molybdenum resistors is described, for example, in U.S. Pat. No. 4,609,903 issued Sept. 2, 1986 to Toyokura et al. U.S. Pat. No. 4,498,228, issued Feb. 12, 1985, to Jillie and Smith discloses a method of manufacturing a Josephson junction integrated circuit including the step of depositing and patterning the resistor material, which typically may be molybdenum, approximately 800 angstroms thick and also having an insulator layer comprising silicon dioxide, approximately 2500 angstroms thick. Jillie et al. note that the first layer of deposited insulator provides an etch stop for the upper niobium etch step. A paper by S. Kosaka et al. entitled "An Integration of All Refractory Josephson Logic LSI Circuit", (IEEE Transactions On Magnetics, volume MAG-21, No. 2, March 1985, pp. 102-109) provides several examples of etch stop by chemical selectivity, i.e., the layer being etched has a much higher etch rate than the layers underneath.